feat: qbz-1 streaming, gapless prefetch, accurate scrobbling, Range-seek

Port proven playback architecture from qbqt fork:
- Bounded VecDeque buffer with condvar backpressure (4MB cap)
- decrypt_and_extract_frames for clean FLAC frame extraction from ISOBMFF
- Cancel+restart seeking with sub-segment sample skipping
- start_prefetch / QueueNext for gapless transitions with pre-started downloads
- track_transitioned signaling for scrobbler during gapless playback
- Range-request HTTP seeking for non-segmented (MP3) tracks
- OnceLock HTTP client singleton with cancel-aware chunked downloads
- Accumulated listening time scrobbling (prevents false scrobbles from seeking)
- Array-format Last.fm scrobble params (artist[0], track[0], etc.)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mainwindow.cpp

@@ -40,8 +40,7 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     m_libraryDock->setObjectName(QStringLiteral("libraryDock"));
     m_libraryDock->setFeatures(QDockWidget::DockWidgetMovable);
     m_libraryDock->setWidget(m_library);
-    m_libraryDock->setMinimumWidth(180);
-    m_library->setFixedWidth(220);
+    m_libraryDock->setMinimumWidth(150);
     addDockWidget(Qt::LeftDockWidgetArea, m_libraryDock);
 
     // ---- Now-playing context dock (left, below library) ----
@@ -70,6 +69,10 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     connect(m_backend, &QobuzBackend::trackFinished,
             m_scrobbler, &LastFmScrobbler::onTrackFinished);
 
+    // Scrobble the finished track during a gapless transition
+    connect(m_backend, &QobuzBackend::trackTransitioned,
+            m_scrobbler, &LastFmScrobbler::onTrackFinished);
+
     // ---- Backend signals ----
     connect(m_backend, &QobuzBackend::loginSuccess,   this, &MainWindow::onLoginSuccess);
     connect(m_backend, &QobuzBackend::loginError,     this, &MainWindow::onLoginError);